1. Field of the Invention
The present invention relates to a recording medium conveying device of a recording apparatus.
2. Description of the Related Art
A printer (recording apparatus) that records desired information such as characters or an image on a sheet-like recording medium such as paper or a film, has been known as an information output apparatus incorporated in word processors, personal computers, facsimiles, and so forth. Various recording methods used in printers are known. Recently, an ink jet recording method has attracted attention because: noncontact recording can be performed on a recording medium such as paper; the operational cost is low; color printing can be easily performed; and it is noiseless because it is a nonimpact method.
Among ink jet recording apparatuses, specifically, a full-line type recording apparatus that has a plurality of recording heads each having a recording element (nozzle) array corresponding to the recording width and that performs recording while conveying a recording medium, is being widely used because faster recording can be performed. Such a full-line type recording apparatus includes a plurality of recording heads that eject different colors of ink, that are arranged in the recording medium conveying direction, and that can eject ink at the same time so as not to reduce the recording speed even during color recording.
The recording heads of such an ink jet recording apparatus have energy generating elements that generate energy to apply to ink for ejecting ink from ejection ports in the form of droplets, and ink flow passages that have the energy generating elements therein and communicate with the ejection ports. Some of the recording heads have an ink container, such as an ink tank, that contains ink to supply to the energy generating elements through the ink flow passages.
Recording apparatuses are configured to record an image (including characters and signs) on a recording material such as paper or a plastic sheet on the basis of recording information. A laser beam printer (hereinafter referred to as “LBP”), an ink jet printer (hereinafter referred to as “IJP”), and a thermal printer have been put to practical use as a substitute for conventional dot-matrix printers. Recently, an LBP capable of high-resolution and high-grade gradation expression and an IJP capable of photo-quality full-color expression have been put to practical use. With increasing performance of IJPs, to fully elicit the recording performance of the recording apparatuses, high-grade recording media such as glossy paper, a glossy film, and coated paper, and even a medium exclusively for photo quality have been put to practical use. Therefore, IJPs are to be performed recording on various types of recording paper, and as fast as an LBP.
High-speed IJPs have full-line type ink jet recording heads and are configured to be able to perform recording just by conveying a recording medium under the recording heads without moving the recording heads in the direction perpendicular to the recording medium conveying direction as in conventional IJPs. Instead of intermittently conveying a recording medium as in conventional IJPs that move recording heads in the direction perpendicular to the recording medium conveying direction, a recording medium can be continuously conveyed, and therefore high-speed recording can be performed. In addition, high-speed IJPs have a much larger number of nozzles for ejecting ink than conventional IJPs, and therefore the recording area per unit time is large. Therefore, by conveying a recording medium at high speed, recording can be performed at sufficient speed.
Such recording apparatuses generally employ a recording medium conveying device that includes an endless belt member configured to convey a recording medium and provided with conductive electrodes and in which the electrodes are charged to generate electrostatic force, the recording medium is attracted to the belt member, and recording is performed while the recording medium is conveyed. The configuration of a known recording medium conveying device will be described with reference to FIGS. 10 to 12. FIG. 10 is a sectional view of an endless conveying belt member. FIG. 11 is a plan view of the conveying belt member shown in FIG. 10. FIG. 12 shows the configuration of a charging unit that charges the conveying belt member shown in FIGS. 10 and 11.
With reference to FIGS. 10 and 11, a conveying belt 31 serving as an endless belt member is integrally provided with an attraction force generator that includes electrodes 36a and 36b formed of conductive metal, a base layer 36c, a surface layer 36d, and voltage receiving members 36e. The voltage receiving members 36e and the surface layer 36d form the same plane. Reference numeral 36e1 denotes voltage receiving members for the electrodes 36a. Reference numeral 36e2 denotes voltage receiving members for the electrodes 36b. The conveying belt 31 is supported by a driving roller 34 and a driven roller 32.
In FIG. 12, reference numeral 35 denotes a tension roller that applies tension to the conveying belt 31.
As shown in FIG. 12, a charging unit that charges the conveying belt 31 has a charging brush 51′, a charging electrode 52′, and a supporting member 53′. The charging brush 51′, which supplies a charge, is in contact with the voltage receiving members 36e. When a charge is supplied from the charging brush 51′ to the voltage receiving members 36e, electrostatic force is generated on the conveying belt 31. Thus, constantly-excellent attraction force can be generated on the conveying belt 31.
On the downstream side of the charging brush 51′ in the recording medium conveying direction is provided a charge eliminating brush (not shown) that comes into contact with the voltage receiving members 36e of the conveying belt 31 and performs charge elimination. By eliminating charge from the conveying belt 31 using this charge eliminating brush and thereby eliminating attraction force, a recording medium can be smoothly separated from the conveying belt 31. Reference numeral 7K denotes an ink jet recording head for black, reference numeral 7C denotes an ink jet recording head for cyan, reference numeral 7M denotes an ink jet recording head for magenta, and reference numeral 7Y denotes an ink jet recording head for yellow.
The above-described known recording medium conveying device is disclosed in Japanese Patent Laid-Open No. 2003-160253.
However, in order to make the endless belt attract a recording medium using electrostatic force, high voltage needs to be applied to the electrodes. In addition, in the case of a recording medium having weak electrostatic attraction force or a recording medium that is hard to attract along the endless belt, the recording medium is hard to bring into close contact with the endless belt, and part of the recording medium is out of contact with the endless belt. In this case, when the recording medium passes under the ink jet recording heads, the recording medium may comes into contact with the bottom surfaces of the ink jet heads and may thereby cause defective recording or damage the recording heads.
The above may be solved by increasing the voltage applied to the electrodes. However, increased voltage is prone to cause leakage of current. The leakage of current may damage the endless belt. If leakage of current occurs between the endless belt and the ink jet recording heads, the leakage of current may damage the ink jet recording heads.
The intensity of electric field generated around the endless belt increases in proportion to the voltage applied to the electrodes. Ink droplets ejected from the ink jet recording heads are susceptible to the electric field. Therefore, ink droplets ejected from the ink jet recording heads are affected by the electric field and miss their predetermined targets on a recording medium. This reduces the quality of the recorded image.